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6 February 2026

12 Pages

Effect of Activated Charcoal Mouthwashes and Repolishing on the Color Stability and Surface Roughness of Resin Composites After Long-Term Water Aging

and
1
Department of Restorative Dentistry, Faculty of Dentistry, Istanbul Atlas University, 34403 Istanbul, Türkiye
2
Department of Restorative Dentistry, Faculty of Dentistry, Istanbul University-Cerrahpaşa, 34098 Istanbul, Türkiye
*
Author to whom correspondence should be addressed.
Appl. Sci.2026, 16(3), 1638;https://doi.org/10.3390/app16031638
This article belongs to the Section Applied Dentistry and Oral Sciences
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Abstract

This in vitro study evaluated the effects of activated charcoal mouthwashes on the color stability and surface roughness of resin composites after long-term water aging and repolishing. One nanofilled (Filtek Universal) and two nanohybrid composites (Optishade Universal, Neo Spectra ST HV) were prepared as disks (n = 12/group). After baseline measurements, specimens were aged in water for 6 months (37 °C), immersed in Colgate Plax White+Charcoal, Natural Mouthwash Charcoal, or distilled water for 12 h, and then repolished. Color change was assessed using CIEDE2000 (ΔE00; acceptability threshold = 1.8) and surface roughness (Ra) by profilometry. Data were analyzed using three-way ANOVA (α = 0.05). Six-month aging caused clinically unacceptable discoloration in all materials (ΔE00 = 6.13–8.87). Charcoal mouthwashes produced partial color recovery, greater with Natural Mouthwash (ΔE00 = 2.85–3.20) than Colgate (ΔE00 = 4.20–5.85), though values remained above the clinical acceptability threshold. After repolishing, only Optishade-Colgate reached an acceptable color. Charcoal increased roughness (Ra = 0.168–0.195 µm), but repolishing restored near-baseline values. Overall, repolishing effectively recovered surface smoothness but provided only limited color recovery, with most materials remaining above clinically acceptable thresholds.

1. Introduction

Growing aesthetic expectations have also influenced dental practice, making tooth whiteness one of the most important determinants of dental aesthetics [1]. White teeth are also widely perceived as an indicator of oral health [2]. However, the consumption of pigmented foods and beverages, exposure to various staining agents, tobacco use, and mouthwashes can lead to tooth discoloration. These multifactorial conditions within the oral environment not only affect natural teeth but also induce physicochemical changes in restorative materials, thereby altering the color and surface properties of dental restorations [3]. Composite resin restorations are widely used in modern restorative dentistry due to their excellent esthetic properties and bonding capabilities [4]. However, these materials are continuously exposed to the complex oral environment, which subjects them to various chemical and mechanical challenges that can compromise their longevity and esthetic outcome [5].
Color stability is a critical parameter for the clinical success and patient satisfaction with composite resin restorations. Factors causing color changes in restorations can be internal or external. Intrinsic factors include the resin matrix composition, the size of the filler particles, the extent of the applied load, and the type of photoinitiator used in the resin. Extrinsic factors include the accumulation of plaque on the teeth, tobacco use, consumption of various foods and beverages that may stain, and the use of certain mouthwashes [6]. Color changes in composite restorations can be quantified using the CIEDE2000 color difference formula (ΔE00), which provides a perceptually uniform measure of color difference. A ΔE00 value exceeding 1.8 is generally considered clinically unacceptable, as it becomes perceptible to the human eye and may compromise the esthetic outcome of the restoration [7].
In addition to color stability, surface roughness plays a crucial role in the clinical performance of composite restorations. Increased surface roughness of dental restorations plays a critical role in bacterial adhesion, plaque accumulation, and surface discoloration. Surface roughness values (Ra) exceeding 0.2 μm have been associated with enhanced biofilm retention and compromised aesthetic outcomes [3]. Surface roughness can develop through mechanical wear from mastication and toothbrushing, as well as chemical degradation from acidic substances in the oral environment [8]. Increased surface roughness not only facilitates bacterial colonization and plaque accumulation but also enhances surface staining, creating a cycle of progressive deterioration in both the esthetic and biological performance of the restoration. Therefore, maintaining smooth surface characteristics is essential for the long-term success of composite restorations [3,5]. When surface degradation occurs, repolishing represents a conservative clinical approach to restore surface smoothness and improve esthetic appearance without requiring complete restoration replacement. This minimally invasive intervention can effectively remove superficial defects and refresh the composite surface, making it a valuable tool in the long-term maintenance of composite restorations.
Mouthwashes are commonly used to control dental biofilm, prevent halitosis and dental caries, and improve tooth whiteness [6,9]. Whitening toothpastes and over-the-counter products such as mouthwashes are in high demand because they are easily accessible and do not require a dental visit [3]. In recent years, activated charcoal has gained significant popularity as an ingredient in oral care products, often marketed as “black magic” for dental whitening [10]. Despite limited scientific evidence, 96% of charcoal-based products in the market claim successful teeth whitening [11,12]. The proposed whitening mechanism relies on the adsorption capacity of activated charcoal, which contains nanopores that facilitate ion exchange and allow it to bind and remove pigments, chromophores, and surface stains [13]. However, concerns exist regarding the potential accumulation of charcoal particles in dental crevices and restoration margins, which may lead to staining [14]. Furthermore, mouthwashes contain various components, such as dyes, detergents, emulsifiers, and organic acids that have the potential to alter both the color and surface properties of composite resins [6,13,15,16].
Research on the effects of charcoal-based mouthwashes on dental restorative materials remains limited despite their widespread use [3,6,13,17,18]. While several studies have evaluated the whitening potential of activated charcoal products on natural enamel, findings have been inconsistent, and whitening effects observed on enamel cannot be directly extrapolated to resin composite restorations, which differ fundamentally in optical properties, water sorption, and chemical structure. The scarcity of clinical and laboratory evidence represents a significant barrier to affirming the safety and efficacy of these products for therapeutic or cosmetic purposes [10,19].
Given the growing popularity of charcoal-containing mouthwashes and the limited evidence regarding their effects on dental restorative materials, this study aimed to evaluate the impact of two commercially available charcoal-based mouthwashes on the color stability and surface roughness of composite resins. Additionally, the effectiveness of repolishing in restoring the color and surface roughness of the materials following immersion was evaluated. The study tested two null hypotheses. The first was that activated charcoal-based mouthwashes do not affect the color stability and surface roughness of resin composite materials. The second was that repolishing does not restore the color stability and surface roughness of resin composite materials after exposure to charcoal-based mouthwashes.

2. Materials and Methods

The composite resin restorative materials and various mouthwashes examined in the present study are detailed in Table 1. Distilled water was used as a control solution throughout the experiments. A priori sample size estimation was performed using G*Power (version 3.1.9.7, Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany). Based on previous in vitro studies evaluating color change and surface roughness of resin composites after immersion in oral care products, a medium effect size (f = 0.25), a significance level of α = 0.05, and a statistical power of 0.90 were assumed for the three-way ANOVA design (Material × Solution × Time). The analysis indicated that a minimum of 10 specimens per experimental group was required. To compensate for potential variability and ensure adequate statistical power, 12 specimens were included in each group.
Table 1. Materials used in the study.

2.1. Sample Preparation

108 disk-shaped specimens (n = 12) were fabricated from three distinct composites, comprising a nanofilled composite resin (FU; Filtek Universal Restorative; 3M ESPE, St. Paul, MN, USA), and two nanohybrid composite resins (OS; Optishade Universal Composite; Kerr Corporation, Brea, CA, USA), (NS; Neo Spectra ST HV, Universal Composite Restorative; Dentsply DeTrey GmbH, Konstanz, Germany) with the A2 shade. The composite resins were placed in a Teflon mold (6 mm × 2 mm) and compressed between glass plates following the application of Mylar strips (Hawe Stopstrip, KerrHawe SA, Bioggio, Switzerland) on the upper surface. The specimens were then polymerized from the top side using a light-emitting diode (LED) curing unit (Elipar S10; 3M ESPE, St. Paul, MN, USA) at a power density of 1200 mW/cm2 for 10 s, per the manufacturer’s instructions. Following polymerization, the specimens were polished using aluminum oxide polishing disks (Sof-Lex, 3M, St. Paul, MN, USA) in the following sequence: coarse, medium, fine, and ultra-fine, each applied at 10,000 rpm for 15 s. The disks were changed every two specimens. All polishing procedures were performed by a single operator using light pressure in accordance with the manufacturer’s instructions. After polishing, to ensure complete polymerization, all specimens were stored in distilled water at 37 °C in 100% humidity for 24 h before further testing. Color measurement and surface roughness of all the specimens were then performed at baseline.

2.2. Color Measurement

All measurements were performed by a single investigator who was not involved in specimen preparation, ensuring blinding to experimental conditions. Color measurements were conducted in accordance with the Commission Internationale de l’Eclariage (CIE) L*, a*, b* color system relative to a standard illuminant D65 on a white background. Utilizing a reflection spectrophotometer (CM-3600A; Konica Minolta, Osaka, Japan) with a wavelength range of 360 to 740 nm, the measurements were facilitated through the SpectraMagic NX software (Ver. 3.40; Konica Minolta, Tokyo, Japan). Before each measurement session, the spectrophotometer was calibrated per the manufacturer’s instructions. The color values (L*, a*, b*) for the upper surface of each specimen were obtained through three measurements, with each sample positioned on the spectrophotometer’s measuring head. Upon completion of three measurements per sample, the device automatically computed the mean values of L*, a*, and b*, which were subsequently documented. The values of color change (ΔE00) for the specimens were calculated using the CIEDE2000 color difference formula, in which ∆L′, ∆C′, and ∆H′ represent the disparities in lightness, chroma, and hue, respectively. SL, SC, and SH denote weighting functions designed to calibrate the overall color difference to accommodate variations in the position of the color difference pair within L′, a′, and b′ coordinates. The factors KL, KC, and KH serve as adjustment parameters for specific experimental conditions. RT functions as a rotational mechanism that considers the interplay between chroma and hue variances in the blue spectrum [20,21]. The ΔE00 values were evaluated utilizing the 50%:50% perceptibility threshold and the 50%:50% acceptability threshold (AT). The perceptibility and clinical acceptability thresholds for color change were established at 0.8 and 1.8, respectively, in accordance with the guidelines of a preceding study [7].
Δ E 00 = ( Δ L ′ k L S L ) 2 + ( Δ C ′ k C S C ) 2 + ( Δ H ′ k H S H ) 2 + R T ( Δ C ′ k C S C ) ( Δ H ′ k H S H )

2.3. Surface Roughness Measurement

The surface roughness was evaluated using a contact profilometer (Surtronic S128, Taylor Hobson, Leicester, UK) with an evaluation length of 1.25 mm and a cut-off value of 0.25 mm. Before measurements, the device was calibrated using a standard plate system (Ra = 0.6 μm). Each sample was measured at three indiscriminate areas and averaged to generate an average roughness value, and was recorded as Ra in μm. The profilometer was calibrated before each new measurement session.

2.4. Immersion Cycle

After conducting the baseline assessments, the specimens for each group were segregated into three distinct subcategories. The surfaces of the samples in each group that would not be measured were numbered, ensuring that the values measured at the beginning and after the experiment belonged to the same samples. The samples were submerged in two mouthwashes, Colgate Plax White+Charcoal (Colgate MW; Colgate-Palmolive, Poland) and Natural Mouthwash Charcoal (Natural MW; The Humble Co. AB, Sweden). Distilled water (DW) served as a control. After color coordinates and surface roughness were measured as baseline, the samples were aged in distilled water for six months at 37 °C in an incubator in 100% humidity. The distilled water was renewed weekly to maintain consistent conditions. After aging, they were kept in 20 mL solutions for 12 h at 37 °C, corresponding to one year, with 2 min of daily use [22]. Specimens were immersed statically (without agitation) in 20 mL of solutions at 37 °C, consistent with standard protocols used in comparable studies [3]. After the immersion periods, the samples were polished again as previously described. Following each immersion period, specimens were rinsed with distilled water and air-dried before measurements.
The analysis was carried out in four distinct phases: initial baseline to establish preliminary values (T0); testing after aging in distilled water for six months (T1); following immersion in charcoal mouthwash (T2); and ultimately after repolishing (T3). This multi-phase evaluation enabled a comprehensive assessment of color stability and surface roughness throughout the study.

2.5. Statistical Analysis

Data normality and homogeneity were verified using the Shapiro–Wilk and Levene tests, respectively. Color change (ΔE00) and surface roughness (Ra) data were analyzed using mixed-design (repeated measures) three-way ANOVA, with Time as the within-subject factor and Material and Solution as between-subject factors. Mauchly’s test of sphericity was conducted to assess the sphericity assumption for the within-subject factor. When the sphericity assumption was violated (p < 0.05), the Greenhouse-Geisser correction was applied to adjust the degrees of freedom. When significant differences were detected, Tukey’s HSD test was applied for pairwise comparisons. A significance level of α = 0.05 was used for all analyses.

3. Results

Mean color change (ΔE00) values and standard deviations of tested composite resin materials in different immersion solutions and after polishing are presented in Table 2. Three-way ANOVA revealed significant main effects for Material (p < 0.001, ηp2 = 0.045), Solution (p < 0.001, ηp2 = 0.197), and Time (p < 0.001, ηp2 = 0.595). Significant two-way interactions were observed for Material × Solution (p < 0.001, ηp2 = 0.027), Material × Time (p < 0.001, ηp2 = 0.023), and Solution × Time (p < 0.001, ηp2 = 0.250). The three-way interaction (Material × Solution × Time) was also statistically significant (p < 0.001, ηp2 = 0.042), indicating complex interdependencies among the factors. Statistical analysis using Tukey’s HSD post hoc test (α = 0.05) revealed distinct patterns of color change across experimental conditions, with groups assigned different letters showing significant differences.
Table 2. Mean color change (ΔE00) values and standard deviations of tested composite resin materials.
After aging in distilled water for 6 months, all composites showed color change above the clinically acceptable value (AT) of 1.8. After rinsing with mouthwash containing activated carbon, a significant level of color recovery was observed in composite resins. Natural MW groups demonstrated the most effective color recovery. Distilled water control groups exhibited minimal changes from the 6-month values.
After repolishing, only the OS-Colgate MW (1.75 ± 1.40) group achieved clinically acceptable color change values below the 1.8 threshold. Most other groups remained above the acceptability threshold despite the repolishing procedure.
Table 3 summarizes the mean surface roughness values (Ra, μm) and standard deviations of tested composite resins for each experimental condition at different time points. Three-way ANOVA demonstrated significant main effects for Material (p < 0.001, ηp2 = 0.004) and Time (p < 0.001, ηp2 = 0.853), while Solution alone did not show a significant main effect (p > 0.05). Significant two-way interactions were observed for Material × Solution (p < 0.001, ηp2 = 0.002) and Material × Time (p < 0.001, ηp2 = 0.004). The Solution × Time interaction was also highly significant (p < 0.001, ηp2 = 0.646). The three-way interaction (Material × Solution × Time) was also significant (p < 0.05, ηp2 = 0.005), indicating complex interdependencies among factors. Tukey’s HSD post hoc test (α = 0.05) was used to identify significant differences between groups, with different uppercase letters indicating temporal differences within material-solution combinations and different lowercase letters indicating solution differences within material-time combinations.
Table 3. Mean surface roughness (Ra, μm) values and standard deviations of tested composite resin materials.
The Ra values of all tested composites remained below the clinically acceptable threshold of 0.2 μm under all experimental conditions. Following mouthwash immersion, surface roughness increased for all composites, with substantially greater changes observed in the charcoal-based mouthwash groups. The Natural Mouthwash groups exhibited Ra values ranging from 0.168 ± 0.044 μm (OS) to 0.180 ± 0.048 μm (Neo Spectra ST HV), whereas the Colgate Plax White+Charcoal groups showed slightly higher values, ranging from 0.188 ± 0.050 μm (NS) to 0.195 ± 0.031 μm (FU). In contrast, distilled water control groups maintained relatively low roughness values (0.055–0.062 µm). Repolishing effectively restored surface smoothness for all groups, with final Ra values ranging from 0.029 to 0.038 µm, approaching baseline levels. Notably, both charcoal mouthwash groups and distilled water groups demonstrated similar surface roughness after repolishing, with no statistically significant differences between solutions at this stage, indicating successful restoration of surface characteristics regardless of prior exposure conditions.

4. Discussion

Dental whitening is one of the most frequently requested treatments for discolored teeth, with over-the-counter products gaining popularity due to their accessibility and cost-effectiveness [1,9]. Recently, activated charcoal-containing products have been introduced into the market for whitening toothpastes, strips, and mouthwashes, marketed as effective whitening agents [1,6]. Despite limited scientific evidence, these products have gained widespread consumer acceptance and are used daily [11,12,13]. The chemical agents used in mouthwashes for whitening purposes can cause discoloration by affecting not only the teeth but also existing restorations [3].
This study evaluated the effects of two activated charcoal-based mouthwashes on the color stability and surface roughness of three resin composite materials following long-term water aging and subsequent repolishing. Composite resin disks were first aged in distilled water for 6 months to simulate long-term oral exposure [23], and then immersed in the test mouthwashes for 12 h, in accordance with previously established protocols [22]. Six months of water aging resulted in pronounced intrinsic discoloration in all tested composites, with ΔE00 values ranging from 6.13 to 8.87 and far exceeding the clinical acceptability threshold of 1.8 [7]. Subsequent exposure to charcoal-based mouthwashes produced statistically significant but incomplete color recovery, with Natural MW groups showing the greatest improvement, while all charcoal groups exhibited a marked increase in surface roughness. Repolishing effectively restored surface smoothness to near-baseline levels across all materials; however, clinically acceptable color values were achieved in only one material-mouthwash combination. Therefore, both null hypotheses were rejected, indicating that charcoal-based mouthwashes significantly affect the optical and surface properties of composite resins and that repolishing, although effective for surface roughness, is insufficient to fully restore color stability in most cases.
Color changes were evaluated using the CIEDE2000 formula, which provides a more precise representation of color perception than the traditional CIE L*, a*, b* system through weighting functions for lightness, chroma, and hue variations [5]. The clinical acceptability threshold (AT) was set at 1.8 ΔE00 as defined by Paravina et al. [7]. Following 6-month water aging, all composites exceeded the clinical acceptability threshold, with ΔE00 values ranging from 6.13 to 8.80. While short-term water immersion studies (24–72 h) [24] and (1 week–1 month) [25] report negligible color changes, longer aging periods consistently demonstrate increased discoloration [6,26,27]. Water molecules diffusing into the resin matrix cause hydrolytic degradation, affecting both organic and inorganic components, leading to filler-matrix interface weakening and matrix softening [28]. This hydrolysis represents a primary intrinsic mechanism of discoloration. Additionally, water absorption increases composite permeability, facilitating absorption of extrinsic chromogenic substances from beverages, foods, and tobacco [3,25]. Discoloration of resin composites involves both surface adsorption and subsurface absorption of chromogenic agents, with the resin matrix composition and polarity influencing the extent of penetration and retention of staining molecules [29]. Our findings align with previous long-term aging studies, which demonstrate substantial color degradation with extended water exposure.
Following charcoal mouthwash immersion, all composites demonstrated significant color recovery, though ΔE00 values remained above the AT. Natural MW groups showed superior recovery compared to Colgate MW groups, with OS-Natural MW achieving the best result (ΔE00 = 2.85 ± 1.48), followed by FU-Natural MW (ΔE00 = 3.12 ± 1.20) and NS-Natural (ΔE00 = 3.20 ± 1.34). Colgate MW groups demonstrated moderate recovery (ΔE00 = 4.20–5.85). While some studies report staining effects of mouthwashes on restorations [6,30], others have documented whitening effects of activated charcoal oral care products [6,17,18]. The mechanism underlying color recovery after charcoal mouthwash immersion in water-aged composites differs from that observed in extrinsically stained specimens. In the absence of external staining agents, the color improvement likely results from: (1) optical masking effects, wherein charcoal particles fill surface microdefects created by hydrolytic degradation, increasing opacity and visually masking underlying discoloration; (2) removal of superficial degradation products accumulated during water aging; and (3) chemical interactions with other mouthwash constituents such as detergents and chelating agents. These findings indicate that the whitening effect of charcoal-based mouthwashes on resin composites is material-dependent, likely reflecting differences in resin matrix composition, filler characteristics, and water sorption behavior among the tested materials.
Several studies have evaluated charcoal products on enamel, with inconsistent findings ranging from minimal effects to modest whitening [9,31,32]. However, enamel findings cannot be extrapolated to composites due to fundamental differences in material properties. Studies evaluating charcoal products on composite restorations have primarily used extrinsically stained specimens and report variable, formulation-dependent outcomes [6,13,17,18,30]. Previous investigations documented partial color recovery in coffee-stained composites, though most did not achieve clinically acceptable levels [13,17]. Our findings extend this evidence by demonstrating that charcoal mouthwashes produce partial color recovery even in water-aged (non-extrinsically stained) composites, likely through mechanisms distinct from chromophore adsorption. The observed material-specific responses align with previous reports indicating that charcoal effects depend on resin matrix composition and filler characteristics [6].
Surface roughness was evaluated using contact profilometry, a widely accepted and reliable method for repeated measurements on the same surface area [33]. Consistent with previous reports, charcoal-based mouthwash immersion significantly increased surface roughness compared with distilled water controls (0.168–0.195 µm vs 0.055–0.062 µm) [3,13]. However, unlike some earlier studies that reported Ra values exceeding the 0.2 µm clinical threshold [3,13], all roughness values in the present study remained below this limit, likely reflecting differences in composite formulations and mouthwash compositions.
The tested mouthwashes were alcohol-free with near-neutral pH, unlike acidic and alcohol-containing formulations that can soften the resin matrix [15], indicating that chemical erosion was not the primary mechanism. The observed roughness changes likely result from interactions between mouthwash constituents (detergents, chelating agents, humectants) and the resin surface, as well as physical retention of charcoal particles within surface microdefects [13,31,34]. Notably, long-term water aging produced marked discoloration (ΔE00 = 6.13–8.87) with minimal roughness changes, whereas charcoal exposure resulted in partial color recovery (ΔE00 = 2.85–5.85) accompanied by increased roughness. This suggests that charcoal acts primarily on the superficial resin layer, improving color while increasing surface texture.
Repolishing is an effective method for removing extrinsic discoloration, and many studies have demonstrated color improvement following repolishing [33,35]. Extrinsic staining occurs through surface pigment accumulation and subsurface pigment absorption [36], and because these changes are largely confined to superficial layers of the composite (<20 μm), they can be removed by polishing. In the present study, repolishing reduced color changes induced by aging and internal water uptake. Some studies further indicate that repolishing can also reduce internal discoloration [35]. The reduction in discoloration observed after repolishing in the distilled water control groups (from ΔE00 = 6.13–7.36 to 3.85–5.20) indicates that a substantial portion of water aging-induced discoloration was confined to the superficial resin layer and could be removed by eliminating the outer 10–20 µm during polishing. However, the persistently high ΔE00 values (3.85–5.20) after repolishing confirm that intrinsic hydrolytic degradation had penetrated deeper into the composite structure, affecting the bulk resin matrix beyond the reach of surface polishing. This demonstrates that long-term water aging causes both superficial and deep-seated color changes, with only the former being reversible through mechanical surface removal. However, despite repolishing, most composite groups still exhibited ΔE00 values above the clinically acceptable threshold. Only one of the nine material-solution combinations, Optishade Universal, exposed to Colgate Plax White + Charcoal (1.75 ± 1.40), reached an acceptable color change, indicating that the effectiveness of repolishing is dependent on both the composite material and the specific exposure conditions. The removal of approximately 10–20 μm of the surface during polishing eliminates part of the organic resin matrix contributing to discoloration; however, deeper internal matrix changes cannot be completely reversed. In contrast to this limited color recovery, repolishing demonstrated excellent effectiveness in restoring surface smoothness, as it removes not only superficially retained charcoal particles and chromogenic deposits but also the chemically altered and softened superficial resin matrix layer, resulting in surface roughness values returning to near-baseline levels regardless of prior mouthwash exposure.
From a clinical perspective, charcoal-based mouthwashes therefore present both benefits and limitations for patients with composite restorations. All surface roughness values remained below the critical 0.2 μm threshold, indicating that concerns related to bacterial adhesion and biofilm accumulation are unlikely, and the excellent restoration of surface smoothness after repolishing (0.029–0.038 μm) demonstrates that roughness increases are readily reversible through routine professional maintenance.
However, esthetic outcomes were less favorable. Despite color recovery during charcoal mouthwash immersion and repolishing, only one of nine material-solution combinations achieved a ΔE00 value below 1.8. Based on these findings, clinicians should counsel patients that: (1) charcoal mouthwashes are safe regarding surface properties and will not increase bacterial adhesion risk; (2) these products provide partial but clinically insufficient color improvement in aged restorations; (3) professional repolishing can maintain surface smoothness but cannot restore acceptable esthetics; (4) restoration replacement remains the definitive solution when clinically significant discoloration is present.
Several limitations must be acknowledged. The in vitro nature may not fully capture the complexity of the oral environment, including salivary flow, enzymatic activity, pH fluctuations, and masticatory forces. The absence of these biological elements may limit the generalizability of the findings to real-world clinical scenarios. The 6-month water aging protocol, while commonly used, represents accelerated aging that may not perfectly replicate gradual clinical degradation. The 12 h mouthwash immersion protocol may not accurately reflect long-term daily use patterns. This study evaluated only three composite materials and two charcoal-based mouthwash formulations, limiting generalizability. Contact profilometry, though reliable, provides two-dimensional measurements rather than comprehensive three-dimensional topography. Additionally, this study relied exclusively on Ra for surface roughness assessment. While Ra facilitates comparison with existing literature, future studies should incorporate complementary parameters such as Rq, Rp, and Rv to provide more comprehensive surface topography characterization. Repolishing was standardized using sequential polishing disks, but clinical techniques vary with clinician experience and restoration accessibility. Future studies should incorporate artificial saliva, thermal cycling, and mechanical loading to better simulate oral conditions, extend observation periods to assess long-term stability, and conduct clinical trials to validate these in vitro findings and assess patient-reported outcomes regarding esthetic satisfaction and oral health.

5. Conclusions

Within the limitations of this in vitro study, both null hypotheses were rejected. Six months of water aging caused clinically unacceptable discoloration in all composites (ΔE00 = 6.13–8.87). Charcoal mouthwashes produced partial color recovery (Natural MW: ΔE00 = 2.85–3.20; Colgate MW: ΔE00 = 4.20–5.85), but all remained above the acceptability threshold. After repolishing, only one material-solution combination, OS-Colgate MW (ΔE00 = 1.75 ± 1.40), achieved a ΔE00 value below the acceptability threshold. Charcoal-based mouthwashes significantly increased surface roughness; however, all Ra values remained below the critical 0.2 μm threshold associated with bacterial adhesion. Repolishing effectively restored surface smoothness to near-baseline levels in all groups. Clinically, these findings indicate that charcoal-based mouthwashes may be safely used with respect to surface roughness but are unable to fully restore the esthetic appearance of aged composite restorations. Therefore, when clinically significant discoloration is present, professional restoration replacement remains necessary to achieve optimal esthetic outcomes. Future clinical studies should evaluate the long-term effects of charcoal mouthwashes on composite restorations under actual oral conditions and assess patient-reported esthetic satisfaction.

Author Contributions

Conception: Z.H.K.; Design: S.S. and Z.H.K.; Data collection: Z.H.K.; Analysis/Interpretation: S.S.; Literature Research: Z.H.K.; Writing Manuscript: S.S. and Z.H.K.; Critical review: S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

No ethical approval was required for this article, as it does not include any studies with human participants or animals conducted by the author.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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